This invention relates in general to digital speech communications, and in particular, to speech encoding using vocoders.
Two-way radios are commonly used in public safety and dispatch operations. Such radios often employ a push-to-talk switch for simplex communication. In a typical operation, an operator engages the push-to-talk switch and begins speaking into a microphone. Voice signals received via the microphone are processed and modulated onto a carrier signal for communication. The push-to-talk switch may be engaged and disengaged several times during a communication session.
Digital voice communication has become commonplace in radio communication systems. Generally, digitized speech is applied to a voice encoder (xe2x80x9cvocoderxe2x80x9d) prior to transmission over a communication link. Modern vocoders use a variety of speech modeling techniques to encode speech, including linear predictive coding, multiband excitation, and others. A vocoder operates to extract speech modeling parameters, such as pitch, voiced/unvoiced classification, spectral amplitudes, gain, and other vocal tract parameters, from the digitized speech. These extracted parameters are encoded to provide a representation of the original speech data. This encoded speech data is transmitted over the communication link. A recipient of the encoded speech data applies a corresponding speech decoder to recover the original speech, which is rendered by a speech synthesizer.
The ability of the vocoder to extract the model parameters required for accurate speech encoding depends in part on the quality of the original speech signal. It is not uncommon for vocoders to include circuitry to remove unwanted signal components, such as signal components resulting from direct current (DC) bias. For example, the improved multiband excitation (IMBE) vocoder used as a standard in the Associated Public-Safety Communications Officers (APCO) 25 standard includes a high pass filter to remove direct current bias from digitized speech signals. This filter includes a feedback network and performs best after a particular elapsed time required for settling and/or stabilization. Thus, the filter requires a particular elapsed time for proper operation.
In many implementations, it is necessary to disable communication circuitry when not in use to reduce current drain. For example, in a simplex push-to-talk two-way radio, there is generally no need to enable the vocoder when the push-to-talk switch is not engaged, as there is no voice input. When the push-to-talk switch is engaged and the vocoder enabled, there may be a small elapsed time before the vocoder circuitry reaches steady state. During such time, the vocoder may be unable to correctly extract model parameters required for speech encoding.
It is desirable to have a vocoder that operates correctly immediately after being enabled such that speech initially processed is properly encoded. Therefore, a new method and apparatus for employing a vocoder in speech processing is needed.